Docs: instruct people to keep using wp-scripts even when extending webpack

[luisherranz]

What?

I've added a note in the wp-scripts docs to ensure people keep using the wp-scripts commands when they extend webpack.

Why?

Because wp-scripts initializes some values (like process.env.WP_SRC_DIRECTORY) that the webpack configuration expects, so it doesn't work when using webpack's CLI directly.

How?

I've added a note in the "Extending the webpack config" section.

[gziolo]

Good idea to remind that it works best when combined with wp-scripts 👍🏻

[luisherranz]

Thanks, Greg!